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arxiv: 2606.20815 · v1 · pith:O3QGRYHKnew · submitted 2026-06-18 · ✦ hep-ph · astro-ph.CO

Warm fermionic dark matter from freeze-in at stronger coupling

Duarte Feiteira , Vin\'icius Oliveira This is my paper

Pith reviewed 2026-06-26 16:25 UTC · model grok-4.3

classification ✦ hep-ph astro-ph.CO
keywords warm dark matterfreeze-inHiggs portalfermionic dark matternon-thermal distributionLyman-alpha constraintreheating temperaturevelocity suppression
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The pith

Fermionic warm dark matter via Higgs-portal freeze-in requires higher reheating temperatures than scalar cases because production is velocity-suppressed.

A machine-rendered reading of the paper's core claim, the machinery that carries it, and where it could break.

The paper studies warm fermionic dark matter produced through freeze-in in the minimal Higgs portal model when the reheating temperature after inflation is low. This keeps the dark matter out of equilibrium even with a larger coupling, opening the door to observable effects such as invisible Higgs decays. The production reaction for fermions is strongly suppressed by the small velocities of the particles involved, so a higher reheating temperature is needed to generate the observed relic abundance compared with scalar dark matter. The resulting momentum distribution of the dark matter is strongly non-thermal and cannot be described by the usual three-parameter fit. Lyman-alpha forest data then rule out masses below roughly 100 to 180 keV, with the exact bound depending on the reheating history.

Core claim

In the minimal Higgs portal, fermionic dark matter production is velocity suppressed, so the correct relic abundance requires larger reheating temperatures than those sufficient for scalar dark matter; the resulting momentum distribution is strongly non-thermal and outside the range captured by the common αβγ parametrization, while Lyman-α observations exclude masses below 100-180 keV depending on reheating history.

What carries the argument

The velocity-suppressed fermionic production reaction in the Higgs portal that enables freeze-in at stronger coupling under low reheating temperatures.

If this is right

  • Larger reheating temperatures are required to match the observed relic abundance.
  • The dark matter momentum distribution is strongly non-thermal and not captured by the αβγ parametrization.
  • Lyman-α forest observations exclude dark matter masses below about 100-180 keV depending on reheating history.
  • Invisible Higgs decays become potentially observable because the coupling can be larger while still satisfying the out-of-equilibrium condition.

Where Pith is reading between the lines

These are editorial extensions of the paper, not claims the author makes directly.

  • The non-thermal distribution could produce distinct signatures in small-scale structure formation beyond the Lyman-α bound.
  • Collider searches for invisible Higgs decays could directly probe the coupling values allowed by this production mechanism.
  • Similar velocity suppression may appear in other fermionic freeze-in models, altering mass bounds across a wider range of portals.

Load-bearing premise

The reheating temperature is low enough that dark matter production from the Standard Model thermal bath remains Boltzmann-suppressed and the dark matter stays out of equilibrium.

What would settle it

A measurement showing that the momentum distribution of warm fermionic dark matter follows the αβγ parametrization or that a particle with mass below 100 keV accounts for the full relic density would contradict the central claim.

Figures

Figures reproduced from arXiv: 2606.20815 by Duarte Feiteira, Vin\'icius Oliveira.

Figure 1
Figure 1. Figure 1: Main contributions to fermionic DM production at low temperatures. [PITH_FULL_IMAGE:figures/full_fig_p004_1.png] view at source ↗
Figure 2
Figure 2. Figure 2: Comoving momentum distribution for TR = 300 MeV, including only the charm quark contribution for an instantaneous reheating temperature profile. The distribution is normalized to unity at its maximum. The result is shown in [PITH_FULL_IMAGE:figures/full_fig_p010_2.png] view at source ↗
Figure 3
Figure 3. Figure 3: Comoving momentum distribution for TR = 300 MeV, including only the charm quark contribution, assuming a flat temperature profile before reheating. The distribution is normalized to unity at its maximum. power law, as shown in [PITH_FULL_IMAGE:figures/full_fig_p011_3.png] view at source ↗
Figure 4
Figure 4. Figure 4: Parameter space of the Higgs portal fermionic DM at low masses, for an instant [PITH_FULL_IMAGE:figures/full_fig_p012_4.png] view at source ↗
Figure 4
Figure 4. Figure 4: 0 2 mc TR 4 6 8 10 q 0 0.2 0.4 0.6 0.8 1 f(q) (a) TR = 0.4 GeV 1 mc TR mτ TR 4 6 8 10 q (b) TR = 0.6 GeV 1 mc TR mτ TR 3 4 mb TR 6 8 10 q (c) TR = 0.8 GeV [PITH_FULL_IMAGE:figures/full_fig_p013_4.png] view at source ↗
Figure 5
Figure 5. Figure 5: Comoving momentum distribution of DM including all production channels, for dif [PITH_FULL_IMAGE:figures/full_fig_p013_5.png] view at source ↗
Figure 6
Figure 6. Figure 6: Parameter space of the Higgs portal fermionic DM at low masses, for a flat temperature [PITH_FULL_IMAGE:figures/full_fig_p014_6.png] view at source ↗
read the original abstract

We study warm fermionic dark matter (DM) in the framework of freeze-in at stronger coupling, in the minimal Higgs portal scenario. The reheating temperature is taken to be low, so that DM production from the Standard Model thermal bath is Boltzmann-suppressed and the DM stays out of equilibrium even for a sizeable coupling. This opens the possibility of observable signatures, in particular invisible Higgs decays. We compute the DM relic abundance including both the pre- and post-reheating contributions. We find that the fermionic DM production reaction is strongly velocity suppressed, requiring larger reheating temperatures than those obtained for scalar DM in order to reproduce the correct relic abundance. The resulting DM momentum distribution is strongly non-thermal and its shape is not captured by the common $\alpha\beta\gamma$-parametrization. We find that the Lyman-$\alpha$ constraint excludes DM masses below about $100 -180\,\mathrm{keV}$, depending on the reheating history.

Editorial analysis

A structured set of objections, weighed in public.

Desk editor's note, referee report, simulated authors' rebuttal, and a circularity audit. Tearing a paper down is the easy half of reading it; the pith above is the substance, this is the friction.

Referee Report

2 major / 2 minor

Summary. The manuscript examines warm fermionic dark matter produced via freeze-in in the minimal Higgs-portal model, taking a low reheating temperature so that production remains Boltzmann-suppressed and the DM stays out of equilibrium despite a sizeable coupling. The relic abundance is computed including both pre- and post-reheating contributions; the fermionic production channel is found to be strongly velocity-suppressed, requiring higher reheating temperatures than for scalar DM to match the observed density. The resulting momentum distribution is strongly non-thermal and is shown not to be captured by the standard αβγ parametrization. Lyman-α constraints are applied, excluding DM masses below 100–180 keV depending on reheating history.

Significance. If the out-of-equilibrium condition is satisfied for the parameter choices needed to reproduce the relic density, the work supplies a concrete, calculable example of warm fermionic DM whose momentum distribution can be directly confronted with structure-formation data and which admits potentially observable invisible Higgs decays. The explicit demonstration that the distribution deviates from the αβγ form is a useful technical contribution.

major comments (2)
  1. [framework condition (abstract and §2)] The central framework assumption—that DM remains out of equilibrium even at the larger couplings and reheating temperatures required by velocity suppression—is load-bearing for both the relic calculation and the momentum distribution used for Lyman-α bounds, yet no explicit verification that Γ_DM(T) < H(T) holds throughout the relevant epoch for the benchmark points is provided.
  2. [§4] §4 (relic abundance computation): the pre- and post-reheating contributions are added after fitting the coupling and T_reh to Ω_DM; it is not shown that the same parameters simultaneously satisfy the out-of-equilibrium requirement used to justify the freeze-in treatment itself.
minor comments (2)
  1. [Lyman-α section] The range 100–180 keV for the Lyman-α bound should be accompanied by a brief statement of the precise velocity-distribution moments or transfer-function cutoff used to obtain the two ends of the interval.
  2. [production rate] Notation for the velocity-suppression factor in the production rate should be defined once in the text rather than only in a figure caption.

Simulated Author's Rebuttal

2 responses · 0 unresolved

We thank the referee for the careful reading and constructive comments. We agree that explicit verification of the out-of-equilibrium condition is required to substantiate the freeze-in treatment for the benchmark points and will add this to the revised manuscript.

read point-by-point responses

  1. Referee: [framework condition (abstract and §2)] The central framework assumption—that DM remains out of equilibrium even at the larger couplings and reheating temperatures required by velocity suppression—is load-bearing for both the relic calculation and the momentum distribution used for Lyman-α bounds, yet no explicit verification that Γ_DM(T) < H(T) holds throughout the relevant epoch for the benchmark points is provided.

    Authors: We agree that an explicit check of Γ_DM(T) < H(T) for the benchmark points is necessary. In the revised version we will add a figure and accompanying text in §2 (or an appendix) showing the ratio Γ_DM(T)/H(T) for the fitted parameters across the production epoch, confirming that the out-of-equilibrium condition is satisfied. revision: yes

  2. Referee: [§4] §4 (relic abundance computation): the pre- and post-reheating contributions are added after fitting the coupling and T_reh to Ω_DM; it is not shown that the same parameters simultaneously satisfy the out-of-equilibrium requirement used to justify the freeze-in treatment itself.

    Authors: We acknowledge that the manuscript does not demonstrate simultaneous satisfaction of the relic-density fit and the out-of-equilibrium condition. We will revise §4 to include this verification, showing that the parameters chosen to reproduce Ω_DM also satisfy Γ_DM(T) < H(T) throughout the relevant temperature range. revision: yes

Circularity Check

0 steps flagged

No significant circularity; central results follow from explicit integration under stated assumptions

full rationale

The paper assumes a low reheating temperature that keeps production Boltzmann-suppressed and DM out of equilibrium (abstract), then integrates the production rate for fermions to obtain the relic density and momentum distribution. The velocity suppression, non-thermal shape, and Lyman-α bound are direct numerical outcomes of that integration rather than redefinitions or self-citations. No equation or claim reduces by construction to a fitted input renamed as a prediction, and the framework condition is an explicit premise rather than a derived result.

Axiom & Free-Parameter Ledger

3 free parameters · 2 axioms · 0 invented entities

The central claim rests on the minimal Higgs-portal Lagrangian, standard Boltzmann-equation treatment of freeze-in, and the assumption of a low reheating temperature that keeps the dark matter out of equilibrium. No new particles or forces are introduced. The coupling strength and reheating temperature are adjusted to match the observed relic density.

free parameters (3)
  • Higgs portal coupling
    Adjusted to reproduce the observed DM relic abundance after including velocity suppression.
  • Reheating temperature
    Chosen low enough to suppress thermal production yet high enough to yield the correct abundance for the chosen mass.
  • DM mass
    Scanned to determine the Lyman-alpha exclusion window of 100-180 keV.
axioms (2)
  • domain assumption Minimal Higgs-portal interaction between the fermion and the SM Higgs doublet
    Invoked to define the production channel and the possibility of invisible Higgs decays.
  • domain assumption Standard Boltzmann suppression at low reheating temperature keeps DM out of equilibrium
    Stated as the condition enabling stronger coupling while remaining in the freeze-in regime.

pith-pipeline@v0.9.1-grok · 5688 in / 1775 out tokens · 24518 ms · 2026-06-26T16:25:57.179850+00:00 · methodology

discussion (0)

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Reference graph

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